In the next generation of wireless systems, it is expected to be the era of multimedia communications. In order to provide such integrated services platform, the system must be able to multiplex users with different transmission rates for different service types. The emerging third generation wireless standards, e.g. UMTS/IMT-2000, are based on Wideband CDMA technology (W-CDMA) to address variable rate requirements of multimedia applications.
In W-CDMA, it is possible to provide multi-rate services by employing the orthogonal variable-spreading-factor (OVSF) codes which can be generated in the form of tree structure. This scheme is known as OVSF-CDMA. The important constraint in OVSF-CDMA is the maintenance of the orthogonality among the assigned OVSF codes. In other words, an OVSF code cannot be assigned to a rate request if the OVSF code has no orthogonality to other OVSF codes already assigned. As a result, the spectral efficiency of the system depends heavily on the code assignment schemes.
In the conventional OVSF-code assignment scheme (CONV), an OVSF code is assigned to a transmission request only when there is an available one that can fulfill the requested rate and the orthogonality is maintained. This scheme has a problem called “code blocking”. The code blocking is the problem that a high-rate request cannot be supported because all supported codes for this rate are not orthogonal to the assigned codes, although the system still has enough capacity to provide the high-rate request. This leads to a high blocking rate for high-rate transmission requests.
To mitigate the code blocking problem, an alternative OVSF code assignment scheme called the region division assignment (RDA) has been proposed. RDA divides the OVSF code tree into regions for each supported rate, and the OVSF codes are reserved beforehand for each region according to the probability of rate requests for each supported rate. By RDA, the users can get the service at the requested rate for at least the already reserved codes. RDA has no control signaling overhead because it has no code reassignment.
The OVSF code assignment algorithm of CONV is outlined as follows. When a request from a user comes, the capacity of system is checked whether it is enough for serving the requested rate. If it is enough, an OVSF code is assigned to the request, provided that there is one that can fulfill the requested rate while the orthogonality is maintained.
RDA uses the strategy of dividing the code tree and reserving the OVSF codes for each supported rate beforehand in order to efficiently assign the OVSF codes for each requested rate later. Therefore, the initialization of the code tree is necessary for the OVSF code reservation.
For a comparison of the performance of CONV and RDA reference is made to “Performance evaluation of orthogonal variable-spreading-factor code assignment schemes based on UMTS/IMT-2000 in W-CDMA”, Vehicular Technology Conference, 2002. Proceedings. VTC 2002-Fall. 2002 IEEE 56th Assarut, R.; Kawanishi, K.; Yamamoto, U.; Onozato, Y. Pages: 1585-1589 vol. 3. This paper compares the performance of the orthogonal-variable-spreading-factor (OVSF) code assignment schemes based on UMTS/IMT-2000 proposed so far.
Further an assignment scheme for orthogonal variable spreading factor code is known from “Nonrearrangeable compact assignment of orthogonal variable spreading factor codes for multi-rate traffic”, Vehicular Technology Conference, 2001. VTC 2001 Fall. IEEE VTS 54th Yang Yang; Yum, T. -S. P. Pages: 938-942 vol. 2.